The present invention relates to a method for controlling the distance of a vehicle to a vehicle traveling ahead, in which the distance and the relative velocity of the vehicle traveling ahead are measured, and the distance is controlled in a proximity-control mode by accelerating or decelerating the vehicle to a preestablished setpoint distance.
Methods and devices of this type are referred to by the name xe2x80x9cadaptive road speed controllerxe2x80x9d as well as by the abbreviation xe2x80x9cACCxe2x80x9d (Adaptive Cruise Control), and they are described, by way of example, in the article, xe2x80x9cAdaptive Cruise Controlxe2x80x94System Aspects and Development Trendsxe2x80x9d by Winner, Witte, et al., published in the SAE 96, Detroit, pp. 26-29, February 1996, Paper No. 961010. Specific aspects of systems of this type are described in German Published Patent Application No. 196 27 727 (corresponding to U.S. Pat. No. 6,311,117), German Published Patent Application No. 196 730 245, and German Published Patent Application No. 196 40 694 (corresponding to U.S. Pat. No. 6,273,204).
In conventional systems, proximity measurement is usually performed by a radar system, which on the basis of the Doppler effect also makes possible a direct measurement of the relative velocity, so that the control system is capable of reacting immediately to measured changes in the velocity of the vehicle traveling ahead.
The setpoint distance, upon which the control is based, corresponds to the safety distance to be maintained in column or convoy travel among vehicles traveling one after the other, and it is dimensioned so that, even in the case of longer vehicle columns and taking into account the reaction times of the drivers involved, rear-end collisions do not result if one vehicle has to brake abruptly due to an unexpected obstacle. This safety distance is a function of velocity and is therefore advantageously determined indirectly by a so-called setpoint time gap, which corresponds to the temporal interval in which vehicles pass the same point one after the other. In an ideal pursuit, the path-time curve of the pursuing vehicle is then the precise image, displaced by the setpoint time gap, of the path-time curve of the vehicle traveling ahead. The same also applies to the velocity-time curve as well as to the acceleration-time curve, it also being possible for the accelerations to have negative values (the deceleration of the vehicle is defined as the amount of negative acceleration).
In practice, inevitable control delays and differences in the vehicle characteristics (acceleration capacity) result in the path-time curve of the pursuing vehicle deviating somewhat from the corresponding curve of the vehicle traveling ahead. Deviations of this type are to a certain extent entirely desirable because they result in somewhat smoothing out speed fluctuations of an xe2x80x9cunsteadyxe2x80x9d vehicle traveling ahead. This smoothing effect can also be strengthened in a controlled manner, for example, by having the accelerations of the vehicle traveling ahead, which are filtered by a low-pass characteristic, accessed by the control system.
An object of the present invention, in a proximity control system of this type, is to improve the comfort and the feeling of safety for the driver and the vehicle occupants.
This objective is achieved according to the present invention as described herein.
In the method according to the present invention, the deceleration permitted in the proximity control process is limited, and in situations in which the setpoint distance cannot be maintained on the basis of this limited deceleration, the proximity controlling process transitions to a process of limiting the proximity to a minimum distance which is smaller than the setpoint distance, and after the minimum distance is reached, the vehicle is further decelerated so that the distance again increases to the setpoint distance.
If the own vehicle approaches at high speed the more slowly moving vehicle traveling ahead, or if, during pursuit, the vehicle traveling ahead is suddenly decelerated, then the method according to the present invention brings about the result that the following vehicle temporarily xe2x80x9cdipsxe2x80x9d into the setpoint distance and then falls back until the setpoint distance is once again attained. In this manner, it may be avoided that the comfort and the feeling of safety are impaired by extreme vehicle decelerations. This dipping strategy, in the method according to the present invention, corresponds to the intuitive behavior of an experienced automobile driver. As a result of the present invention, the behavior of the control system is therefore brought to more closely approximate the natural behavior of a human automobile driver, and in the process irritations are avoided which may otherwise result from the different behavior of the automatic control system. The temporary undershooting of the setpoint distance is unobjectionable from the safety technical point of view because it is only brief, and the assumption may be made that the driver is very alert as a result of the braking. In the traffic-technical sense, the method according to the present invention has the effect that velocity fluctuations or abrupt velocity changes are far more powerfully cushioned than would be possible using a simple proximity control system. It is conventional that velocity fluctuations of this type, especially in heavy traffic buildups on a highway, may be amplified in a regressive wave and may finally lead to a traffic jam. In this regard, the present invention also contributes to the flow of traffic and therefore ultimately to traffic safety.
The minimum distance to the vehicle traveling ahead, which is not supposed to be undershot even in a dipping process, may be described by a time gap, which is designated as the dipping time gap and which is smaller than the setpoint time gap. The minimum distance is then the product of dipping time gap and velocity of the own (following) vehicle. The difference between the actual distance and the minimum distance is the deceleration distance (measured as the relative distance between the vehicles), within which, in response to dipping, the relative velocity between the vehicles may be reduced to zero. From this deceleration distance and the known instantaneous relative velocity, an acceleration value may be calculated using the path-time law for a uniformly accelerated motion, the acceleration value assuring that the relative velocity is actually reduced within the deceleration distance. However, this only applies under the assumption that the vehicle traveling ahead maintains its velocity at a constant value. If this is not the case, the calculated acceleration value may also have added to it thexe2x80x94if necessary, appropriately filteredxe2x80x94acceleration of the vehicle traveling ahead. On the basis of the acceleration obtained in this manner, it is possible to control the dipping process so that the minimum distance is not undershot, and the acceleration of the own vehicle in its amount remains as small as possible.
In principle, it is sufficient to determine the deceleration distance and the acceleration derived therefrom only once at the beginning of the dipping process and then, in the further course of the dipping process, to take into account only the accelerations of the vehicle traveling ahead. The velocity changes of the vehicle traveling ahead, however, may be even further cushioned if the deceleration distance and the acceleration derived therefrom are also continually actualized during the dipping process. However, to prevent the deceleration distance from becoming excessively small or from declining to zero, which may result in unrealistically high deceleration values, the deceleration distance in this case may be limited to a positive minimum value. In determining the deceleration distance, abrupt transitions may be avoided by performing interpolations in a proximity range below the setpoint distance, between the lower threshold value and the theoretical deceleration distance, which is yielded by the actual distance, the instantaneous vehicle velocity, and the setpoint time gap.
When, at the end of the dipping phase, the minimum distance to the vehicle traveling ahead has been attained, then the own vehicle may be further decelerated so that the distance again grows to the setpoint distance. This may be achieved using control technology by basing the calculation of the necessary acceleration from the deceleration distance and the relative velocity not on the actual relative velocity but rather by adding to this relative velocity an appropriate return velocity. The system behaves as if the velocity of the vehicle traveling ahead is smaller by the return velocity than the actual velocity. This has the consequence that the deceleration of the own vehicle does not end in a relative velocity of zero but rather ends in a positive relative velocity corresponding to the return velocity, so that the own vehicle again falls back to the setpoint distance.
When the setpoint distance is reached, it is possible to switch back to the normal proximity control system. However, with reference to a gentle transition between the different control modes, it is possible through additive superimposition to create a setpoint value from the acceleration values that are generated from the proximity control process and from the proximity limiting process, the setpoint value then being supplied to the engine control system. In the context of the proximity control process, in this case the value range of the permissible setpoint accelerations is limited so that only setpoint accelerations above a predetermined negative threshold value are generated, whereas in the proximity limiting process the value range of the accelerations is limited by an upper threshold value, for example, zero. By adding these setpoint values, the result is then a fluid transition between proximity limiting and proximity control.
Further advantages are yielded from the following descriptions of example embodiments.